The only difference you will see here , looking at the OEM parts is one starter is Staggered and the other is not all fit; Big/Small Block, GM 90 Degree V6 ; no mention of auto or standard/manual trans.

NOT TRUE

Manual trans cars/trucks and Auto trans cars/trucks use different starters.
Manual trans starters have noses that are cast iron and a bit thinner than the AT versions.
The AT starter will not even fit into most manual trans bell housings.

If you are using a 621 bell for the Muncie----you should get a starter for a manual trans vehicle----suggest a starter for a 70 Chevelle with a manual trans.

If you are using a truck bellhousing, you will need to get a spacer ring for the big bore in the bell housing as the bearing retainer on the Muncie is too small for the hole in the truck bell.
Spacers are available from McCleod Clutches---also some transmission vendors are offering a larger bearing retainer for the Muncie to go into a truck bell.